Sky's Foreign Affairs Editor Sam Kiley said the base was an important capture for the Russians.

"It is a base that is home to a significant number, possibly a third, of the main combat aircraft of the Ukrainian air force - the MIG-29s - and their support aircraft and the structures that go with them.

"If you look at that and the blockage of the Ukrainian Navy in the shared port of Sevastopol in Crimea, what you see here is the Russians doing two things.

"The first is to seize territory that they now lay claim to and the second is to cripple the Ukrainian armed forces.

"That is extremely important to them if they want to move into the Ukrainian eastern provinces where there are a predominance of Russian speakers."

The Foreign Office has extended its travel warnings and is advising against all but essential travel to Kharkiv, Donetsk and Lugansk due to increased tension in eastern Ukraine.

Video:Putin Signs Crimea Annex Treaty

At the Kremlin on Friday, Russian President Vladimir Putin signed legislation incorporating Crimea into Russia and hailed it as a "remarkable event".

International monitors are due to arrive in Ukraine in the next 24 hours to examine the political and security situation in the country.

Russia agreed the move - claiming it would help stop "nationalist bandits" in the country - but has barred the observers from going to Crimea.

Video:Klitschko On Taking Crimea Back

It also hit back at the widening of sanctions, calling them "divorced from reality" and said it reserved the right to impose sanctions of its own.

The 57 member countries of the Organisation for Security and Co-operation in Europe (OSCE) agreed an initial deployment of 100 monitors to regions in the east, south and west of Ukraine.

They will spend six months in the country and 400 more could be added "as necessary and according to the situation", diplomats said.

Video:EU To Reduce Use Of Russian Energy

Western countries have been pushing hard for an observer mission as a way of preventing an escalation of tensions in Ukraine following Russia's annexation of the Crimean peninsula.

Russia had blocked the plan on previous occasions.

OSCE vice-chairperson Thomas Greminger welcomed the decision as a "very meaningful contribution to de-escalate the situation in Ukraine".

Video:Ukrainian PM Arseniy Yatsenyuk

But, in a statement on Saturday, Russia's Foreign Ministry made it clear Crimea is a 'no-go area' for the observers.

It said: "The mission's mandate reflects the new political and legal realities and does not apply to Crimea and Sevastopol, which became a part of Russia."

"Russia hopes that the objective and impartial work of the international observers will help to overcome the internal Ukrainian crisis, stop rampant nationalist banditry, eradicate ultra-radical tendencies."

Video:EU And Ukraine Sign Landmark Deal

Prime Minister David Cameron and other EU leaders have also imposed travel bans and asset freezes on 12 more people to punish Moscow for its takeover of the Ukrainian territory.

There are now 33 Ukrainians and Russians on the list, accused of playing some part in what Western powers say is an illegal land grab of the region.

Russia said the move was necessary to prevent oppression of Russian-speaking people on the peninsula following the Ukrainian political crisis which saw President Viktor Yanukovych flee the country.